Karen Allen was never shown a complete script before she worked on this film. Director William Friedkin deliberately kept her in the dark, since her character Nancy wasn't supposed to be aware of what was happening to Al Pacino's cop as he explored the gay underworld.

In 1972 director William Friedkin-huge after The French Connection (1971)--was shooting his spiritual/psych-horror The Exorcist (1973) in downtown New York. For a scene requiring mock brain-scans of the possessed lead character, he shot a real-life radiologist and his assistant, Paul Bateson. In 1979 Friedkin was planning an adaptation of Gerald Walker's novel "Cruising", inspired by a real-life serial killer who was carving up "leather boys" in the city's underground gay bars and dumping their body parts in the Hudson River, wrapped in black plastic bags. When Friedkin learned that his "Exorcist" radiologist assistant Bateson was awaiting trial for the post-coital slaying of gay film critic Addison Verrill, Friedkin decided to pay him a visit to do a little research into the psyche of his cruising killer. Bateson was later sentenced to life in prison for the Verrill murder, but not before dropping hints while in custody that he was also the body bag killer. Those cases remain unsolved, but there's a good chance that Friedkin had not only inadvertently consulted the actual killer responsible for the murders that were the subject of both the novel and the film it was based on, but that Friedkin had also cast him in a film he made years before.

Rumor has it that William Friedkin's original cut ran 140 minutes. There is an increasing demand for a home video release of this version, if the deleted footage still exists and was not, as commonly believed, destroyed by United Artists. Apparently, Friedkin delivered this cut to the studio and it was condemned by the MPAA for ultra-provocative content. Friedkin has said that all of the footage he was ultimately forced to cut (over a series of 40 different edits, at a cost of $50,000, before the MPAA finally granted the film an R rating) involved graphic sexuality and wasn't connected to the murder investigation plot, and that the footage did provide some "twists and turns in the story" that were not present in the film's final cut.

In 1977 and '78 the gay community in New York City was terrorized by a series of "bag murders"--six male victims were murdered, mutilated and dismembered, their remains wrapped in black plastic bags and dumped in the Hudson River. Some of the grisly fragments washed up on the New Jersey shore, others came to ground near the World Trade Center. Police traced items of recovered clothing to a shop in Greenwich Village, catering to gays, and distinctive tattoos identified one of the victims as a well known member of the gay community. Because several of the cases involved unidentified persons and there was no confirmed cause of death, the crimes were not officially classified as homicides but were listed as CUPPI's--circumstances undetermined pending police investigation. One of the cases was solved due to evidence collected in an "unrelated" case. On September 14, 1977, film critic Addison Verrill was beaten and stabbed to death in his New York apartment. Charged with the slaying, Paul Bateson, a 38-year-old X-ray technician, confessed to meeting Verrill in a Greenwich Village gay bar. After having sex at Verrill's flat, Bateson admitted to crushing his victim's skull with a metal skillet, afterward stabbing Verrill in the heart. Convicted of the homicide on March 5, 1979, he was sentenced to a term of 20 years to life in prison. While in custody awaiting trial, Bateson bragged of killing other men "for fun," dismembering their bodies and dropping the bagged remains in the Hudson River. This case inspired the novel upon which this film was based. Detectives were satisfied that Bateson actually was the serial killer they had been looking for, but lack of solid evidence resulted in his not being charged with them.

In an effort to make the scenes of gay "leather bars" as real as possible, actual leather bars in New York City that catered to gay men were used. These locations are in Manhattan's lower West Side neighborhoods in the meat packing district, most of which remain open to this day. The extras in full black leather and chaps, among other provocative sexual attire, were actual patrons of the bars recruited for the scenes a few days before filming began. They were instructed to act as they would normally act in gay bars, but to tone down sexually-oriented activities because of the likelihood these acts would give the film an "X" rating.

Angered over reports of the film's negative portrayal of gay life that leaked out during shooting, activists distributed hundreds of whistles to members of New York City's gay community and encouraged them to spoil exterior filming on city streets by blowing them loudly whenever they spotted a crew from the film shooting on location, a tactic that cost the studio a considerable amount of money that had to be spent on lost production time and post-dubbing.

Joe Spinell made this movie before Maniac (1980), playing a brutal, closeted police officer in the early scenes. His character mentions his wife leaving him and moving down to Florida with their young daughter to live with his wife's sister. In real life, Spinell's wife, porn star Jean Jennings, divorced him several weeks before he filmed his scenes and moved to Florida--exactly as his character describes.

The L.A. punk band The Germs recorded five or so songs expressly for the soundtrack to this movie, although only "Lion's Share" was actually used. During the recording sessions, director William Friedkin was so energized by the Germs' playing that he took to doing the "pogo" dance around the engineer's booth.

During a murder scene in a room at the St. James Hotel where a man is stabbed to death on a bed, director William Friedkin edited in several near-subliminal frames from a gay hardcore movie that can clearly be seen in slow motion on DVD.

In addition to protests that occurred when the film was released, the production itself was plagued by demonstrations protesting it. Protesters would clog streets, make lots of noise--in order to ruin live recorded sound--and even climb up on rooftops and shine lights with reflectors down on to the set,k to disrupt the lighting and distract the crew.

The plot, about murders of the patrons of gay nightclubs, caused many to protest the release of the film. In fact, organized protests by gay groups were planned on the day the film was released. The public largely ignored the protests.

William Friedkin returned to the project after some well-publicized murders were committed in New York City's gay community were committed. He remembered the project that The French Connection (1971) producer Philip D'Antoni had first talked to him about a few years earlier, which eventually resulted in this film being made.

William Friedkin initially wasn't interested in the project when producer Philip D'Antoni first brought it to his attention. D'Antoni then tried Steven Spielberg who dropped out when--not surprisingly--none of the major studios were prepared to back it.

Throughout the summer of 1979, members of New York's gay community protested against the production of the film. Gay people were urged to disrupt filming and gay-owned businesses to bar the filmmakers from their premises. People attempted to interfere with shooting by pointing mirrors from rooftops to ruin lighting for scenes, blasting whistles and air horns near locations, and playing loud music. One thousand protesters marched through the East Village demanding the city withdraw support for the film. Al Pacino said that he understood the protests but insisted that upon reading the screenplay he never at any point felt that the film was anti-gay. He said that the leather bars were "just a fragment of the gay community, the same way the Mafia is a fragment of Italian-American life", referring to The Godfather (1972), and that he would "never want to do anything to harm the gay community".

William Friedkin asked noted gay author John Rechy--some of whose works were set in the same milieu as the film--to screen the film just before its release. Rechy had written an essay defending Friedkin's right to make the film, although not defending the film itself. At Rechy's suggestion, Friedkin deleted a scene showing the Gay Liberation slogan "We Are Everywhere" as graffiti on a wall just before the first body part is pulled from the river, and added a disclaimer: "This film is not intended as an indictment of the homosexual world. It is set in one small segment of that world, which is not meant to be representative of the whole." Friedkin later claimed that it was the MPAA and United Artists that required the disclaimer, calling it "part of the dark bargain that was made to get the film released at all" and "a sop to organized gay rights groups". Friedkin claimed that no one involved in making the film thought it would be considered as representative of the entire gay community, but gay film historian Vito Russo disputes that, citing the disclaimer as "an admission of guilt. What director would make such a statement if he truly believed that his film would not be taken to be representative of the whole?"